[llvm-dev] Why can't we devirtulize this?

[Sameer Abu Asal via llvm-dev]

Hi,

I came across this case that shows a bad case for de-virtualizeation

https://godbolt.org/z/e7r6a8

If you have a call to a virtual function inside of a Loop, llvm is unable
to de-virt the call simply with Instr Combine. However, for the case where
we put the virtual call inside a loop.

You can recreate the output for inst-combine with:

clang++ -mllvm -print-after-all -mllvm
-filter-print-funcs=_Z10devirt_badm -S -O3 -emit-llvm
./test_devirt.cpp -mllvm -debug-only=instcombine -o ./test_devi
rt.ll |& tee /tmp/log.bad

clang++ -mllvm -print-after-all -mllvm
-filter-print-funcs=_Z11devirt_goodv -S -O3 -emit-llvm
./test_devirt.cpp -mllvm -debug-only=instcombine -o ./test
_devirt.ll |& tee /tmp/log.good

It seems to me a problem with Inst combine not being able to hoist the call
to get the virtual func pointer outside of the loop, is that right? Can we
possibly do better than this?

Thank you,

--Sameer
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